1. Field of the Invention
The present invention relates to a semiconductor light emitting device such as a light emitting diode or a semiconductor laser, and in particular, to a semiconductor light emitting device having an improved light extraction efficiency, and a method for manufacturing the semiconductor light emitting device.
2. Description of the Related Art
In order to improve the light extraction efficiency of a semiconductor light emitting device, attempts have been made to form a regular structure (concaves and convexes) of a nanometer size on a front surface of the light emitting device (Applied Physics Letters, 142, vol. 78, 2001, Jpn. J. Appl. Phys., L735, vol139, 2000). Since the period of the concaves and convexes has a nanometer size, the concave and convex region behaves like a layer that smoothly varies the refractive indices from the surface of the semiconductor to air. Thus, no reflection occurs and the light is completely transmitted through the semiconductor.
However, with such a structure, the light extraction efficiency varies greatly depending on the shape of the concave and convex region. Consequently, the structure has not produced sufficient effects. Further, the above regular structure must be produced by drawing based on electron light, etching, or the like. Thus, the fabrication of the structure requires high manufacturing costs and has a low productivity. Therefore, the structure is not practical. Moreover, since the regular structure must be produced so as to have a nanometer size, the margin of the process is low.
Further, in order to form a structure of a nanometer size on the surface of the light emitting device, the surface is etched using a resin composition to form a micro phase separation structure as a mask. After selectively removal of one of the phases of the micro phase separation structure, a base surface is etched using the other phase as an etching mask (Jpn. Pat. Appln. Publication No. 2003-258296). In another known method, the front surface of the light emitting device is roughened by treating the surface using hydrochloric acid, sulfuric acid, hydrogen peroxide, or their mixture (Jpn. Pat. Appln. Publication No. 2000-299494).
However, these methods are susceptible to the crystal face of the substrate. That is, some exposed faces can be roughened but others cannot, depending on the orientation of the crystal face. Thus, the light extracted surface cannot be always roughened, thus limiting the improvement of the light extraction efficiency. Further, the shape of the concaves and convexes significantly affects the light extraction efficiency. The desired shape has not been identified so far.
As described above, in order to improve the light extraction efficiency, fine concave and convex structures on the surface of the semiconductor light emitting device have been attempted to form. However, sufficient effects have not been obtained because optimum shapes of the concave and convex structures were unknown. Furthermore, it is very difficult to highly reproducibly form fine concave and convex structures on the surface of the light emitting surface.